Process for acidizing injection wells



, 5,804,145 f PROCESS Fonnorm-zine lNJECTlON .iO rrin c. llolbrook, Crystal Lake, 11L, assignor m The v on NoDrawing Application November Serial N 472,223

team cr esnoff- This invention is directedto stimulating the recovery of residual oil from partially depleted 'reservoirsQlt is especially concerned with the selective acid treatingof formations surrounding injection secondary recovery operatioins. o

As, a well-completion or stimulation technique, the

acid treatmentof geological" formations to reduce theirresistance to the fipw of fluids therethrough has become- 1 a well established procedure. Although'used extensively in the treatment of producing formations to facilitate pnmary'recoviery of the fluids contained, therein-,itis also employed. in the treatmentof' partially depleted fluid reservoirs wherein thej ndig'enous fluidcnpulsi'on forces have diminished to theex'tent that it is uneconomical to. employ conventional primary recovery 'methods. Thev recovery of fluids from these partially depleted reservoirs is generally termed secondaryrecovery and etfectcd by a va-riety 'of eXpedients Perhaps the most commonly usedmethod involvesthe artificial injection of a fluid from an injection well into the; formation-to carry along' or drive, ahead residual'oil' through the reservoir and force it into a producing well 'or well-s Both gas and:

water have been used for'injecti'on purposes. The choice of fluid depends upon a number of .factors, such as iithOe logicalv characteristics, availabilitynf gas orwater, manner-in which the residual oil is held in the reservoir, i. e.,

I adherent or capillary -held oil, properties of the residual oil, etc. Wherea fiuid is employed as the driving means.

in the secondary recovery operations, it is desirable that g the formation have a suitable injectivity index. This in:

dex is a criterion .of the capacity'o f a well to'pass in je'cted fluids into the reservoir rock. Quite-frequently it is necessary to treat the formation surrounding'an in- WELLS:

Pure Oil Company, Chicago, Ill, a corporation-of 1 wells employed in f' treat the less permeable streaks of a partiallydepleted,

1o y from formations having a non-uniform permeability by available, including-the so-called two-pump method}? permeability streaks. In manner the by-passing' of- "the acid into more'permeable adjacent zones-is at'l'east'f minimized. However within the Zones or intervals, which are isolated there may also be streaks of low perpAc'cording to this invention, this desiderat'um may be known as transient -back'-pre ssure,

, jection well with a formationpenetrator, i. 'e., acidizing. v or hydraulic fracturing, to alter;the existing inje'ctivity characteristics a'swellas provide aunifor'm distribution of the water or gas being'ini qcted into the formation, Structural characteristicsoli geological formations vary. 1 Accordingly, the. following distinctions will beused in 1 describing the subjectinvention: f

- A fQrmation is any sedimentary bed or stratum sufiicicntly homogeneous to be regarded as a unit. Fluids such 1 as petroleum oil, gas, and/ or water may be found dis" tributed throughout the formation or only in intervals or. zones of the formation. Although. a formation may be substantially homogeneous in composition, formations do occur which vary transversely in permeability. Wherethe variations in formations are relatively thin they are referredto as streaks.v There are frequently encountered intervals or zones in the formation withalternating orsuccessiye streaks of varying permeability "wherein different ratios of more permeable-streaks to less permeable Istreaksareencountered... a general rule' it would be perferred if the formation penetrator was introduced into 4 those streaks having a permeability. of less thariabout '75 millidarcies. The selective directing of the fluid treating, agent into the less permeable. streaks is adyantageous because maximum enhanxment of. ilow tion penetrator suc has acidizing'or hydraulic fracturing. Q This and other objects will be made more apparent by the.

' Tarysealing or plugging agents have also been employed. In general, these techniques permit the acid or other entireroneor interval of a. producing formation is sealed.

. only the more permeable streaks -occurring within a more slowly. A low permeability streak can-keep-a backf pressure for several'days, while the back-pressure inithe' in a short period of tim'e, In carrying out the instant jization of the latter sections of the formation maybe ac- Accordingly, if the sealant is injected between the time when the transient back-pressure existing in the less the well bore, the sealant will preferentially be forced into the more permeable parts oi the formation. For example sealing or plugging agent is introduced into the fluid is introduced into the formationin the sol phase and;

' mined by volumetric displacement measurements or the injection pressure. Because of the transient back-presume it is therefore an object of this invention to selectively fluid-bearing formation, surrounding an injection well used in thewatenfioodin g of said formation with a forrna following detailed description of this invention .fl In increasing the etficiency of oil and gas production selective acidizing techniques, a number of expedients are jet-acidizing'- with" jet acid-guns,- used of packers, and temporary bridge plugs which shut otf subjacent zone: where acid treatment is not wanted. In addition, temporchemical, treatment of an entire zone or' inte rval,sel'ectedbecause of its ratiooi high permeability streaks to lowmeability which-ishould be treated in ;o rder to increase their permeability.-

however, is not possibl'e'using conventional. isolation methods noted. above wherein a Because of the presence of alternating higlipermeability and low permeabilitystreaks, it is' desirable to isolate producingzone in order that more extensive intervals can be acidizedwith minimum amounts of treating reagents,

effected by taking advantage of a pressure phenomenon This phenomenon manifests itself when the pressure employed ingthe injection of waterinto" a' partially depleted reservoir durtion near the well bore diminishes to formation pressure quite rapidly. However, the pressure in the leS.S P'firl'ne able parts of the formation is retained and decreases much rnore' permeablestreaks decreases to formation pressure? invention, the isolation of the less permeablestreaks front the more permeable streaks to facilitate the selective acid complished by'employing' the back-pressure in the less; premeable parts of the formationto prevent the inimical penetration of sealant into the said less permeable streakse,

when. the injection pressure is first reduced and the time permeable streaks decreases to the pressure existing in.

during themurse of injecting the drive fluid, e. g., water,

into the ,formation, a suitable amount of a temporary,

strearnat normal injection pressure. "The sealant em ployed in the instant invention is a reversible "gel which then gelates. When the sealing agent has been positionedadjacent-the formation interval to be treated, fas deter like, the injection 'pressure is substantiallyreduced; and

the formation interval being'treated is contacted with the sealant at. apressure-substantially lower than normal I sealantwill enter the more permeable streaks and solidify therein. The acid or other substance, fluil, usedin treating the less permeable streaks 1s thenv injected into the less permeable streaks, which'were' not porary sealing agent is then removed from the formation interstices-in which. it was disposed and, if necessary, the acid is back-flowed. Thereafter, the water injection is resumed;

7 detail the practic'ot this invention; I 4 A water injection well penetrating a calcareous formacore analysis data, has 500 p. s. i. at a rateof 110 producing formation,' which.

chloric acid treatment, is about-22 feet thickand at a and rate, after equilibrium flooding conditions have attained', about 50 gallons oftemporary sealant, consisting of a 2%v weighfpercent s'olution of gumltaraya in a i into the fl'oodihg water. Pumping of the resultingmixsealant and the flooding water is positioned atthe top of the 22 footsection, which is to be treated. The injection pressure at the wel p. s. Land the pumping. sealant has been forced The water flood is shut continued until the temporary off after the sealant has been 1.000 gallons of 15% pumped into the unseal wellhead injection pressure'of about 300 p. s.'i. The ire j V jection of the flooding water is continued after the a-cidiz' log 'phase has been completed. will be diverted through the streaks which have beeir acidizedf However, after about 15-30 hours the temporarysealant disposed "become peptized, permitting the liquefied sealant to'be flushed from within The 'fiood waferwill then Partially fiow through these flooding agent. cable streaks occ bution rate of the ing of 'the less perm lustrated ,by the foregoing example." However, the subject inventio'rl is not limited by the materials, condi Itions or techniques employed. V vA variety of sealing agents may be. utilized in carrying out the instant invention. The sealant employed is of a.

of'nliethods. lt preferred whico is used as the temporary sealant b'e self-executing. Bodiedsubstances of this nature may be prepared by thickeninga solvent such various strong inorganic acids, e. g., hydrochloric, sulfuricfete, or organic 'solvents, e.- g.,- light petroleum distillates, low molecular et c., with a water soluble gum such as gum garayajtjraga canth, galbanum, etch rkb out l- 1- 0 parts' of gum per 7 199-90 parts of solvent" will provide a suitably thickened.

without further treatment. 1 Further details onthe charexisting in. tireless permeable streaks, the temporary ture is continued until theJlower interface betweerithe I head is then reduced to 250-=i into the several .loose streaks. flushed from the well bore into the fo'rmation, and about bywe'ight hydrochloric acid is" ed. streaks in thcsformation at a:

within the several loose streaks will the interstices of the loose streaks.

V streaks, thereby efi'ecting an improvementin the distri The selective acidiz urrin'gina producing interval of a partially depleted reservoir has been ilf kes-44s 1 such as fracturing actenstics of gninvcontainin 4 g tem found in US; Patent 2,595,131; a r r Other typesof. gelling a cuts may also be used to p. pare the reversible gels employed in this. invention a include the aluminum soaps derived from long-chain fa acids such as the so-called napalm soaps which are m tures of aluminum l'aurate and aluminum soaps of cycl parail'inic and unsaturated'acids. The introduction" sufiicient amounts, preferably l 1 0 parts per 99-90 pa; of a suitable solvent, will provide a gel which will 'revl v tothe fluid stage up'onhcing'contacted witha -peptiz The following homlimiting example shows in greater tion which has several loosesti'caks, as determined by .a normal injection pressure of J barrels of water/day. The

is susceptible to a hydroformation depth of ISSZ-feet has a temperature'of about 85 F. 'While maintaining the normal injection pressure 1 15% by weight solutioh of hydrochloric acid, is injected i that the peptizing of the gelweight chlorinated hydrocarbops,- faro ma-tic hydrocarbons,

substance which maybe employedasa temporarysealant. Gels prepared employing acidic solvents will peptize I types ofsubstances which jection' system maybe 40 Initially the flood waterfments of tubing and prior art for: permitting theinjection of the drive-fluid.

. P Y d, although solutions of fluophdsph tion or the formation niques. However, it

such as water, ammonia or substituted ammonia col pounds,- strong acids, and others. These peptizers" m: be incorporated in the'gelled sealant so that the visoosi of the sealing agent will be substantially reduced aft a time delay sufli'cient to permit treatment of the 'streal withinjthe formation, ,which remain unsealed during tl -carrying out of this invention.- Suitable-solvents inclut light petroleum distillates, animal and vegetable oils, an matic hydrocarbons, etc. More completedetail on tl characteristics of this type of temporary sealant is four in U. S. Patent 2,596,844". r I In the event that reversi state is not effected by the terials, gelling agents such disposed in a suitable so! on to the, low viscosity flui use of suitable'peptizing m: as high molecular weight soar vent maybe employed as sea ing agents.'- These gels are heat'reversible; After the: sealants have functioned in their intended capacity th more permeable streaks coptainingsuch sealants may b opened byith'e application of heat from a suitable sonic:

It isevident that the foregoing discussion of severe I may .be 'used in carrying on the instant invention is only illustrative. Other reversibl gels which'may be readily reverted to the less viscous stat will be obvious to those skilled in this artand as such an considered within the scope of this 'invention.-

in carrying out the solation of the less permeabl streaks within a producing. interval, a conventional in employed with slight m'odifica in water-drive operations must be "suitably equipped liefore'the drive-fluid isintroduced. Anumber of arrange packers have been described in'tht In general, howeventhe well is equipped so thatthe inj'ectionof thedri-ve-fluidis confined to the formation or formations being treated, *This' also facilitates theinjeetion of the sealing agent and acidizing materiah j f The subject invention is applicable to the treatment of calcareous, dolomitic, or siliceous formations with forma' .tion penetrat ors to effect .an increase in the permeability. In the event that an acidizi'ng agent is employed as the formation penetrator, the type of formation will determine the'typ'e of treating reagent which is to be en}- ployed. In the case of calcareous or dolomitic form'stions, hydrochloric acid solutions are preferably em-. other mineral. acids, such as: sulfuric; nitric, e'tc.,may be utilized. In the treatment of siliceous formations, any reagent capable of reacting with thefsiliceous co'nstituents of the formation to improve theage characteristics thereof isto be employed. Aqueous oric acids arepreferred. Other hydrogen fl uoridecontain'ing or hydrogen fluoridepio ducing reagents may also-be utilized. As in conventional acidizing practices, the-amount of acid .ern'ployed' will depend upon the formation extent, permeability characteris tics, and other lithologicalconsiderations. The acidiz'asubsequent to the isolation-of the. from the more less permeable streaks permeable is carried out employing'conve ntiona'l acidizing tech-- is preferred to employ a system' wherein themofi ififlafion ofi the fluidIinjee 'on-pystem-ii v Petroleum ln'stitntejmso).

from piping changes;

in-described function as peptizing agentsf'the acidizing phase of this invention may be carried out effectively without unsealing the more permeable. streaks containing,

the temporary sealants of this invention.- -The rate of ditfusi'on of acid through the sealant is negligible and any fpeptizing of the sealant will be local and occur at the exposed face. Accordingly, the peptizing of the sealant V with the acidizing agent be slow enough to permit the hydraulic fracturingtechniques in the treatment of the less permeable streaks to'incre'ase their permeability.

, Where fracturingfiuids are' employed as the. formation penetrator, conventional hydraulicffracturing techniques V are employed. Processes wherein a'fracture is produced ll 78. formation by applying s'utficient hydraulic pressureby means of a low viscosity pressure. transmitting medium I are comprehensively discussed in the prior art. For eit ample's'of suitable-processes see U. S, Patents, Re. 23,'

- 2 24, 2,'6s 1 ,4s1,-ana others. Y r

, Accordingly, the use of acidiaing techniquesand hydraulic fracturing processes in that phase'of this invention directed to. increasing the permeability of the unsealed, 1

. less; permeable streaks will be ref erred in the appended claims as the use of formation penetrators.

During the injection of the gas or water into the for-mas tion, it may be desirable, just prior to carrying out the selective'acidiiation processor.- this invention, to increase the injection pressure in excess of that normally employedin order ihat a greater transient back-pressure may be developed inthe less permeable streaks of the formation- Prior tocarrying out the selective acidization, it is dey sirable that complete and accurate information-concerninglthe lithological: and stratigraphic properties of theproducing formation interval be obtained. However, one of the advantages of the instant invention is that a loggingequipment such a s that employed for measuring the spontaneous-potential of formations may be used for logging permeability variations Although the time ele I held to a. minimum to avoid treatment may beestriblishedj v The exact time, however which the sealing. agent will preieren'tia lly enter the more permeable streaks depend upon initial in jection pressure-,penncability; viscosity of the liquefied sealing agent, as well as a number of other conditions which must be' taken intoconsideration before the selective acidization-is employed.

It is preferred in carrying out the instant invention that the reduction in pressure, necessary to eliec't a suflicient 10 transient back-pressure in the less permeable streaks and prevent an ingress of inimical amountsof sealant therein be about one-half of the normal injection pressure." It concomitant acidtreatment of the unsealed less permeable;

'. It is also within thesscopeof this invention to employ is obvious, however, that the normal injection pressure he reduced sutficiently to produce not only the necessary transient back-pressure but also to provide enough pressure to force the temporary sealant into the more pen of injecting the formation penetrant into the less perfmeabl'e' streaks 'to effect an increase "in the permeability 7 thereof. In selecting the normal inj'e'ctionpressur'e care must be exercised; to avoid pressure-partihg of the formaftion being treated, rln'general, a pressure about 0.5 to

1.5 .p. s. i./foot of well depth of pressure is applied. For example, in ihe Bradford field in northern Pennsylvania where oil is produced from formations 1100-2100 feet below the surface well, head-prcs sure sof as high as 1600 p. s.' i. have been employed. In the Bartlesville ,{Saltd areas of northern Oklahoma, well headpressures of about 600 p. st'i. vare employed in water flood sands en-:

countered at a depth of about 1200 feet.

"Although the foregoingdiscuss iou'was limited to application of the instant invention to the selective acidization of partially depleted fluid-bearing reservoirs which are being drained by secondary rccovery'ruethods,

7 it is also possible to employ the instant invention in the selective acidization of'formationswhich. are being eX- ploited by primary recovery methods. Thus oil, gas, or

water-bearing formations may be treated in this manner.

minimum amount of preliminaryinvestigat'iqn of the formation to. be treated is necessary. Conventional wellment-can be ascertained experimentally ina laboratory by means of bench scale equipment, a more pract calap-' proach utilizes correlation of pressure-decline at the Wellhead with time. This indicates-the relative amount of streaks of varying permeability because the higher the.

proportion or streaks of low permeability the slower the "pressure decline will be. For example, infstudies of formation streaks having a wide variation in permeability,

.a formation having a permeability of 6 millidarcies will have an intake rate of 0.85 barrel of watr/day/foot ata well head injection pressure of about 600 p. s. i. If v the" well head pressure is reduced to about 300 p. s. i. the

intake rateis'reduced' to 0 for about one hour, slowly increasing thereafter until: equilibrium flow conditions 7 I occur after about 24 hours. O'n the other hand, a formation having a permeability of millidarcies, would have.

an intake rate 'of 6.0 barrels of water/day/foot at the 'initial injection wellhead pressure of about 600 p. s'.- i.-

" However, when the injection pressure is-reduced to 3 p. s. i., the how conditions, although affected return to "equilibrium'in less than one. hour (Vide Secondary Re c'bvcry of Oil in the United States, 2nd ed., American particular streak to decrease to normal formation pressure can be determined,.and in this manner. the timingof the, sealing and acidizing phases of the selective acid ..In these instances it is only necessary to equip the well bore for the injection of a liquid into the formation. The transient back-pressure can be produced therein, in the same manneras described above; Whilewater may be employed, in order to avoid emulsification and 'sepa'r-atiqn problems, it is preferred that the lease oil; i, c., crude oil, p'troleum,'distillateoils such as kerosene,gas oil,- etc., or someother. fluid compatible with the crude oil being produced from the reservoir be employed. In addition, other types of injection liquids may be used and their selection will depend upon theneeds and judgment of those skilled in this art. The acidization phase ofthe invention can be carried out in the manner described tively acidizing the less permeable streaks of said forma tion traversed by an injection-well which comprises in:

I 'l 'he time required for the transient back-pressure of a jecting al-iq'uid into said formation at. a pressure substantiallyin' excess of'normal formation pressure, reducing said pressure substantially below :a'rnbient formation PICQ? sure whereby a transient back-presure is produced in said formation by the bnckflow of the injected fluid into said injection ,well, injecting into said injection well a teu'1- porary sealing agent'after the transient baclepress'ure in the more permeable streaks'has dissipated to .a pressure suflicient to permit the introduction-of said"sealiug agent into said more pen ricable streaks whereby the-transient bac-k-pressure retnainingin said formation is isuflicicut to prevent the inimical penetration of said sealing agent "c ao-si ica agent within thelstzeakspenetrated thereby for a time I sufficient to effect't be solidification. of the sealing agent, and thereafter forcing intothe unsealed streaks an acidic formation penetrator capable of reacting with the con- Y stiiuents of' said formation in an amount sufficient to inv I of thesealing agent into said more permeable streaks streaksg crease the permeability of said unsealed, less permeable ambient formation pressure whereby a transient back pressure is produced in said formation by the outflow therefrom of theinjected liquid, injecting into said injec- 21A method in accordance with claim 1- in which may liquid injected into said formation to inducescid transientback-pressure is a petroleum oil.

3; A method in acco dance withclaim 1 in which the back-pressure is water. l.t

4. In the treatment of geological fluid-bearing formaliquid injected into s'aid formation to induce said transient ,tions having mama; permeability difi'erentials resulting fromv theoccurrenee within said formation of adjacent streaks of varying permeability, said formation being ex} ploited by secondary recovery means employing fluid inection, a method for seleeuvely acld treating the less 7 permeable streaks of said reservoir which comprises injccting into said formation, which is traversed by an in! iection well; a liquid at a pressure sutficientto force said liquid into-the interstitial passages ofsaid f'ormatiomreducing said pressure below the ambient formation pressure whereby a transient back-pressure'is produced in said formationlby the backflow into. said' injection well I of* the injected liquid, injecting into said injection well a temporary sealing agent after the tra-nsient backpressure in the morepermeable streaks hasdissipated to streaks penetrated thereby' for a time sufiicient to effect the solidification of said sealing agent, introducing into the tion .well a temporary sealing agent after the transient. back-pressure in the more permeable streaks has dissi patcd to a pressure sufiicient ,to permit the introductionwhereby the transient back-pressure remaining in said formation' is su-fiicient to prevent the inimical penetration of said sealing agent into the less permeable streaks,., maintaining the sealing agent within the more permeable unsealed, 'lcss permeable streaks .an acidic formation penetrator capable'of reacting with the constituents of said formation in an amount sufficient to increase the perrncabllity of said less permeable streak-s and at a pressure i intermediate" between the said injection pressure and the said reduced pressure at which the temporary sealing agent was injected into'the formation, removing the temporary sealin'g'agent from the more permeable streaks, and ther" after introducing into' said reservoir additional amounts of injection fluid at a pressure and in amount sufficient a press'ure'sufficient to permit the introduction of the seal-' ing agent into said more permeable streaks whereby thetransient back-pressure remaining in s'a id formation is sufiicient to preventthe inimical penetrationof said sealing agent into the less permeable streaks, maintaining the sealing agent within the more permeable streaks penejtrated thereby for a time sufficient to effect the 'solidifica-' tionpf said sealing agent, and thereafter forcing into the unsealed streak-s an acidic formation penetrator capable of reacting with the constituents of said formation in amount suificient toinc'reasethe permeability of said unsealed, less permeablestreakst;

41 5. A method in accordance with claim 4'irt'which the temporary sealing agent is a reversible gel capable of being pcptizedto substantially reduce its viscosity; a

6. A method in accordance with claim 5 in which said I I Ireversiblc gel is an acidic solution of a-water-soluble gum in a strong mineral acid.

7. ln the treatment of geological fluid-bearing formations-havingintra'zone permeability differentials resultingfrom the occurrence within said" reservoir of adjacent' streaks of varying permeability, said reservoir being exploited by secondary recovery means employing fluid injection, a method for selectively acid treating theless permeable streaks of said reservoir which comprises in-- ecting into said formation a liquid ata pressure sufi'icient to force saidliquid into the interstitial passages of said formation, reducing said pressure substantially below to induce drainage 'of the residual oil in said reservoir.

' tion of said temporary sealing agent into said formation is injectio n, .a method for selectively acid treating the less and in an amount sufiiccnt to increase the permeability of 8. A method in accordance with claim 7 in which said liquid is injected into said formation'at a pressure ofabout 0.5-1.5 p. s. i./foot of 'welldepth and the injeceffected at about .6 the pressure at which said fluid is introducedin-tosaid format-ion. I 9. In the treatment of geological fiuidzbearing forma- 'tions having intrazone permeability differentials resulting a from the occurrence within said reservoirof adjacent streaks of varying permeability, said reservoir being exploited by secondary recovery means employing fiuidpermeable streaks of said reservoir which comprises injecting into said reservoir water at a pressure between. about 0.5 to 1.5 p. s. i./foot of well depth to induce the drainage of said reservoir, reducing said pressure to about /2-" of said injection pressure, contacting said formation l with an acidiesolution of gum'karaya ,in hydrochloric acid, forcing said acidic solution at said reduced pressure l 1 i l into the more permeable streaks whereby said more per-' tf meable streaks are temporarily sealed, forcing an-aeiclizittg medium into the unsealed streaksat a pressure intermediate the said injection pressure and said reduced pressure said reduced streaks, removing saidsealing agent from said more permeable streaks and thereafter introducing additional amounts of said water to induce further drainage-of the residual oil in said reservoir:

References in the file of this patentl- Z I V STATES PATENTS 2,402,588 Andresen Jun=2 9 Mar. 27, 1951 

1. IN THE TREATMENT OF GEOLOGICAL FLUID-BEARING FORMATIONS HAVING INTRAZONE PERMEABILITY DIFFERENTIALS RESULTING FROM THE OCCURRENCE WITHIN SAID FORMATION OF ADJACENT STREAKS OF VARYING PERMEABILITY, A METHOD FOR SELECTIVELY ACIDIZING THE LESS PERMEABLE STREAKS OF SAID FORMATION TRAVERSED BY AN INJECTION WELL WHICH COMPRISES INJECTING A LIQUID INTO SAID FORMATION AR A PRESSURE SUBSTANTIALLY IN EXCESS OF NORMAL FORMATION PRESSURE, REDUCING SAID PRESSURE SUBSTANTIALLY BELOW AMBIENT FORMATION PRESSURE WHEREBY A TRANSIENT BACK-PRESSURE IS PRODUCED IN SAID FORMATION BY THE BACKFLOW OF THE INJECTED FLUID INTO SAID INJECTION WELL, INJECTING INTO SAID INJECTION WELL A TEMPORARY SEALING AGENT AFTER THE TRANSIENT BACK-PRESSURE IN THE MORE PERMEABLE STREAKS HAS DISSIPATED TO A PRESSURE SUFFICIENT TO PERMIT THE INTRODUCTION OF SAID SEALING AGENT INTO SAID MORE PERMEABLE STREAKS WHEREBY THE TRANSIENT BACK-PRESSURE REMAINING IN SAID FORMATION IS SUFFICIENT TO PREVENT THE INIMICAL PENETRATION OF SAID SEALING AGENT INTO THE LESS PERMEABLE STREAKS, MAINTAINING THE SEALING AGENT WITHIN THE STREAKS PENETRATED THEREBY FOR A TIME SUFFICIENT TO EFFECT THE SOLIDIFICATION OF THE SEALING AGENT, AND THEREAFTER FORCING INTO THE UNSEALED STREAKS AN ACIDIC FORMATION PENETRATOR CAPABLE OF REACTING WITH THE CONSTITUENTS OF SAID FORMATION IN AN AMOUNT SUFFICIENT TO INCREASE THE PERMEABILITY OF SAID UNSEALED, LESS PERMEABLE STREAKS. 